In transmission systems for heavy-duty vehicles, for example trucks and buses, it is known to connect an extra gearbox to the basic gearbox of the vehicle for the purpose of doubling the number of gearing possibilities (see, for example, SE 453379—the equivalent of which is U.S. Pat. No. 4,821,591 and WO 9620359; both U.S. Pat. No. 4,821,591 and WO 9620359 are published in English and are hereby expressly incorporated herein by reference for purposes of disclosure). Such a gearbox is usually referred to as a range gearbox or an auxiliary gearbox. The range gearbox usually comprises a planetary gear, by means of which the gearing can be changed between a high-range mode and a low-range mode. In the low-range mode, use is made of the gearing in the planetary gear, whereas, in the high-range mode, no transmission takes place through the planetary gear. In order to facilitate shifting between high-range mode and low-range mode, it is known to design such planetary gears with synchronizing means, usually comprising synchronizing rings, spring means for pre-synchronizing and blocking means in order to prevent engagement before synchronous rotational speed has been achieved.
In order to reduce the number of component parts, and also from the point of view of power distribution, it has been found to be advantageous to use the ring gear of the planetary gear as a coupling sleeve. The ring gear can, by axial displacement and after synchronization of the rotational speed difference between the ring gear and coupling rings, be brought into engagement with coupling rings located on either side of the planetary gear, and, in this way, different gearings are obtained. Examples of known art with an axially displaceable ring gear are shown in SE514231—the equivalent of which is US 23110876A1 publication and which is hereby expressly incorporated herein by reference for purposes of disclosure. In this case, the ring gear is provided at each axial end with synchronizing means, and the internal teeth of the ring gear are extended so as to be capable of being coupled together with a corresponding coupling ring with coupling teeth arranged on each side of the ring gear. The ring gear and the synchronizing means on each side thus occupy more space in the axial direction.
When the ring gear in SE 514231 (US 23110876A1) is coupled together with the gearbox casing, low-range mode is obtained, and the gearing in the planetary gear is used.
In order to obtain a shorter shifting time when synchronizing the rotational speed for the low-range mode, a greater synchronizing torque is required compared with synchronization for the high-range mode. A large diameter of the low-range synchronizing device is thus desirable in order to obtain a greater synchronizing torque. The diameter refers to the position of the synchronizing friction surfaces. In the range gearbox according to SE 514231 (US 23110876A1), the inside diameter of the ring gear limits the possibility of a large synchronizing diameter and thus a great synchronizing torque. Another disadvantage of known art is that the overall length is in many cases too great and that double sets of annular springs with associated synchronizing rings are required.
An object of the present invention is to make synchronization for the low-range mode act on as large a diameter as possible in order to obtain more rapid shifts and also to shorten the overall length of the gearbox and reduce the number of component annular springs and synchronizing rings, while retaining good function. An object is also, when the axial displacement of the ring gear takes place, to separate friction surfaces in the synchronizing device with a relative speed difference; that is to say the synchronizing device which is not functioning at the time, and in this way reduce the friction losses.